metamorphic
Gneiss
Gneiss
Hardness: 6-7 Mohs; Color: Pink, gray, white, and black; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 Mohs
- Color
- Pink, gray, white, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Pink, gray, white, and black; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under intense heat and pressure. Most gneiss is Precambrian or Paleozoic in age.
Uses & applications
Used in construction as crushed stone for road bases, as dimension stone for architectural facings, countertops, and landscaping.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The banding is caused by the segregation of light minerals like quartz/feldspar and dark minerals like biotite/amphibole.
Field identification & locations
Identified by its characteristic alternating bands of light and dark minerals. Common in the roots of mountain ranges and in shield areas such as the Canadian Shield or the Appalachian Mountains.
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